This invention relates to the prevention of bit balling while drilling wells through fine formation materials such as sticking clays.
A problem is encountered in drilling through earth formations which contain materials which stick to the .[.drill bit or well pipe.]. .Iadd.drill string. This usually occurs on cutting devices, such as drill bits or reamers, or at other discontinuities in the diameter of the drill string, such as where stabilizers are inserted. .Iaddend.If sufficient formation materials stick to the drill bit its cutting action can be greatly reduced and the drill string can become stuck in the hole. In the event the drill string becomes stuck, certain procedures can be employed for freeing the pipe. Initially, there is an attempt to pull loose with the elevators. Next there are jars located in the drill string that are utilized in an attempt to jar loose from the formation. If the drill string cannot be extricated by the elevators or because of the lack or ineffectiveness of jars in the drill string then more expensive procedures must be employed. An attempt is usually made to remove all the pipe located above that stuck on the formation. The free point is determined and an explosive charge is utilized to loosen the connection located directly above the free point. Once the connection is loosened, the pipe above the free point can be rotated to separate it from the pipe still stuck in the hole. Once the drill pipe above the free point has been removed, wash pipe is run into the wellbore which is of a larger diameter than the stuck drill pipe remaining in the hole. When the wash pipe has been lowered over the drill pipe water is circulated down the wash pipe and up the annulus in an attempt to wash away the material binding the drill pipe. When it has been determined that the remaining portion of the drill string has been freed from the formation, a fishing tool is run into the wellbore in an attempt to spear the top joint of drill pipe in order to retrieve the drill string from the wellbore. If either the drill pipe cannot be freed by the wash over process or it cannot be retrieved with fishing tools, whipstocking up hole must be utilized in order to bypass this area. A deflecting surface is located above the stuck pipe and a new drill string is run into the hole and deflected past the stuck pipe.
It can readily be seen that these procedures are expensive and great care should be taken to avoid having stuck drill pipe. The cost of these procedures is a great deal higher if sticking is encountered in offshore wells. Even if bit balling does not result in becoming stuck, the reduced drill rate plus the necessity to pull the drill string to clean the bit and collars prove to be quite costly.
Additionally, bit balling can cause blow outs if the drill string is pulled and a high pressure zone has been drilled. When the drill string is pulled the balled up drill bit acts as a swab on the formation and causes it to blow out. Because of the tremendous cost involved with taming a blow out, great care is exercised to prevent excessive bit balling in such situations. Accordingly, more trips are made to clean and check the bit when sticking formations are drilled. Thus, the effective drilling rate is greatly reduced increasing substantially the cost of the well.
One formation material which proves most troublesome is shale composed of montmorillonite clay. This clay has two tetrahedral silica layers and a central octahedral alumina layer. This clay exhibits a large negative charge. Exchangeable cations neutralize this negative charge and exists in a cation swarm at the surface of the clay. When these cations are fully hydrated they are more loosely bonded to the clay due to its distance from the clay surface caused by the large radius of the hydrated molecule. The further these cations are from the clay, the weaker is the bond between the cations and the clay.
During a drilling operation a water base mud is generally used especially in offshore areas where oil base muds are impractical due to anti-pollution rules. When the drill bit contacts these clays the circulating water base mud acts to further hydrate the cations adjacent the clay. As the drill bit contacts the clays, the hydrated cations permit easier shearing apart of clay particles. Once these clays have been separated from the remaining clay matrix, they exist in a partially dispersed hydrated state where they have their maximum plasticity. Because the clays exhibit a large negative charge they stick to the metallic drill string upon contact. In this state the clays will build up on the drill string and will eventually disrupt the drilling operation.
One method of combatting these sticking clays is to use an oil base mud having a water phase saturated with sodium and calcium chlorides. These chlorides control the clays by osmotic pressure, where water contained in the clay will transfer to the chloride saturated water phase of the oil base mud in attempting to equalize the salt concentrations.
In offshore areas, however, oil base muds are risky to use because of anti-pollution rules. In these offshore areas, lime or gyp mud systems are frequently used to inhibit the clay swelling. The lime or gyp system prevents further hydration of the clays and often removes water held by the clays similar to the process described in the discussion of oil base muds. These lime or gyp systems however, can be fairly expensive due to dilution of the mud requiring extensive addition of mud material to keep up the mud weight. In those areas where oil base muds can be used, the greater expense of such muds over normal drilling fluid makes it desirous to find a less expensive drilling system for sticking clays.
It is therefore an object of the present invention to provide an improved drilling system for use in drilling through sticking earth formations.